The present invention relates to apparatus for coating a moving web of paper, and in particular to an improved short dwell time applicator having a replaceable orifice plate.
Conventional applicators of the trailing blade type include means for applying coating material to a paper web that is usually supported and carried by a resilient backing roll. Such applicators may include a chamber having an opening extending across and parallel to the web, together with a doctor blade located on a trailing side of the opening, which serves to meter and level the coating, and a front wall extending from a leading side of the opening toward the web. Means are provided for supplying a coating liquid to the chamber, and thence through the chamber opening and into an application zone between the front wall and doctor blade and onto the web. To seal the end spaces between the front wall and doctor blade to prevent escape of coating material laterally of the web, edge dam means are provided thereat.
In recent years it has become desirable to produce printed papers having a minimal amount of coating, i.e., on the order of about two or three pounds of coating per ream of paper. In order to achieve low coat weights on conventional trailing blade equipment, it is necessary to increase the pressure of the doctor blade against the web, which results in a high rate of wear on the blade and necessitates more frequent replacement of the blade. High blade pressure also increases the possibility of web breakage and streaking caused by foreign particles being caught between the blade and web.
Many conventional coaters inherently employ a relatively long dwell or soak time, which is the time interval between initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersible materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive wet coats without intervening drying because the successive coats tend to migrate into and contaminate the previous coat.
To overcome the disadvantages of prior applicators and produce printed papers having a minimal amount of coating, there has recently been developed an improved applicator, as taught in Damrau et al U.S. Pat. No. 4,250,211, assigned to the assignee of the present invention. The applicator disclosed in that patent is commonly referred to as a short dwell time applicator, and utilizes a reservoir of liquid coating material established in an elongate and narrow application zone between a forward liquid coating material seal and a rearward doctor blade. The coating liquid in the zone is maintained under pressure for pressurized application on the web, and after application is almost instantaneously wiped by the doctor blade. The liquid seal closes the forward end of the zone so that a pressure of coating material may be developed therein, and is defined within a gap between an upper end of an orifice plate and the web, the arrangement being such that a copious excess of coating material introduced into the zone flows through and fills the gap to form the seal.
The size of the gap determines the rate of flow of coating material therethrough and therefore the integrity of the liquid seal and the pressure of material in the application zone, and the orifice plate is adjustable in height to control gap size in response to changes in the viscosity of coating material as well as in applicator head angle. The orifice plate is not readily removable from the applicator, so that adjustment of gap size is made by moving the orifice plate toward and away from the web with a screwjack through a series of linkages and cam followers. Such an adjustment technique has proven to be complicated and costly to fabricate, and difficult to maintain.